Conventionally, the outer lens portion of automobile lamp assemblies whether used on the rear, front or side surfaces thereof, have been divided into two or more distinct sections. At least one of these sections includes optical elements which transmit light from a light source in the assembly outwardly thereof and another section made up of reflex elements the purpose of which is to reflect light from an exterior light source directed at the lamp assembly. As is well known, these reflex elements or reflectors are quite useful at night because they can allow drivers to easily spot other vehicles and increase driving safety.
Referring to FIGS. 1A and 1B, there is shown an example of an automobile 1 with its front lights 3 being directed toward the tailgate lights 5 of another automobile 7. As can be seen, an incident beam of light 10 is redirected by a reflector 11 inside the tailgate lights 5 of the second automobile 7 and it comes back towards the driver in the first automobile 3 as a reflected beam of light 12.
Referring to FIG. 1C, the design of the reflectors 11 is based on the retroreflector principle inside plastic prisms, which are typically hexagonal. These reflectors 11 are mandatory in automobile vehicles and their light signal is standardized in various countries. The reflectors 11 are formed by the juxtaposition of prisms of small dimensions, in all the surface of the reflective zone. The side length of each prism is generally inferior to 4 mm.
Referring to FIG. 2, there is shown a portion of a molded plastic retroreflector 13 made according a known plastic injection process that involves injecting a single layer of plastic 9 on a matrix mold made of a multiplicity of prisms. Only one prism 14 is illustrated in FIG. 2 for simplicity. The side length of this prism is shown by distance “d”.
The Applicant has discovered that molded reflectors with prisms of more than 4 mm in side length are prone to shrinkage problems. Indeed, shrink marks 15, which are shown in dotted lines in FIG. 2, are created during the cooling of the plastic after the injection step and this affects the reflective properties of the prisms.
As is known in the art, the fabrication of a plastic reflector requires numerous steps. The fabrication by plastic injection on a mold matrix called an electroform first requires the fabrication of metal prisms.
Referring to FIG. 5, the prisms are fabricated from elongated metallic rods 6 that have a geometrical configuration such as a hexagon. At the end of each rod, three faces are machined. These faces are called the reflecting faces and are all symmetrical. The angles of these faces are optimized so as to conform with the standards of light reflections for vehicles. The side length of hexagonal prisms mostly varies between 2 mm to 4 mm. Above 4 mm, these prisms are considered “large prisms”.
Once these prisms are fabricated, they are assembled by juxtaposition one next to the other according to a certain configuration that is required by a particular retroreflector. Some optic elements that are non-reflecting can be inserted between them.
Once the assembly of prisms is made, it is deposited in an electrodeposition bath so as to obtain a negative of the geometric configuration. This negative piece is called the electroform or mold matrix.
It is the electroform or mold matrix that is placed in the injection mold. The purpose of the electroform or mold matrix is to permit to the plastic that will be filling the injection mold to take the same shape as the geometric assembly of retroreflector prisms that was made during the previous steps. The plastic is transparent and allows light to travel through it so that the faces of the prisms return light in an opposite direction. The plastic may be made of polymethyl methacrylate or polycarbonate and can have different colors.
The molding parameters are numerous in an injection device. These can be adjusted so as to obtain an esthetically and functionally satisfying piece. In the field of retroreflectors, the goal is to obtain a plastic piece as similar as possible to the metallic prisms that are used to make the electroform or matrix mold.
The shrink marks 15 that on the retroreflector 13 are the result of known problem in the injection process of plastic and are due to a non-uniformity of the thickness of the plastic that is injected. The shrinkage of the piece depends on its thickness and takes places during the cooling of the plastic. In this case, the thickness of the plastic varies between a minimum thickness of “e1” and a maximum thickness of “e2” corresponding to the cavity of the prism, where “e2” is much larger than “e1”. Therefore, since the thickness of the plastic is not constant, the faces of the prisms are deformed. It is not possible to maintain a constant thickness on a reflector since one side has to form the geometry of the prisms and the other side has to be flat in order to correctly return the light. The deformation of the faces affects the reflecting properties of the prisms, thus lowering the photometric values of the reflectors. Up to now, one solution to this problem has been to modify certain injection parameters during the injection process in order to mold reflectors up to 4 mm in prism side length.
However, larger prisms that have a side length superior to 4 mm cannot be molded in the conventional manner because of the shrink marks that appear on the reflecting faces. These shrink marks affect the photometric values and do not satisfy the standards. These “large prisms” are more in demand because of the style that they provide. Indeed, the reflex zone does not result in a reflecting surface made of numerous small luminous points, but it is rather made of large luminous points that can be disposed as desired on the lens.
Therefore, there exists in the market a need for an injection process for forming a retroreflector of a larger size but that does not suffer from shrinkage problems that could affect the reflection properties of the prisms. In particular, this process would allow one to mold retroreflector prisms whose side lengths are larger than 4 mm.